1. Field of the Invention:
The present invention relates to a process for removing dissolved oxygen (hereinafter referred to as DO) from water and particularly to a process for removing DO present in a low concentration (1 mg/l or less) from water and a system therefor.
The present invention can be utilized for rinsing water used in the electronics industry, feedwater for boilers, water used in various steps of the food industry, cleaning water, etc.
2. Prior Art:
The conventional processes for removing DO from water include physical processes (e.g., thermal degassing, vacuum degassing, purging with N.sub.2 gas) and chemical processes involving the addition of a reducing agent.
The process of thermal degassing comprises contacting a gas-containing water directly with steam to heat the water and thereby reducing the water solubility of the gas to remove DO from the water. This process is employed mainly at power plants requiring a large amount of water, in order to remove DO from feedwater for boilers to prevent the corrosion of boilers, etc.
The process by vacuum degassing is used mainly for degassing of cooling water. The vacuum degassing tower used in the process is packed with Raschig rings, etc.; in this tower, water is sprayed at the top to increase the surface area of the water present in the tower and further a vacuum is created inside the tower, whereby the gas dissolved in water is discharged out of the tower. A membrane degassing process which effects vacuum degassing via a hydrophobic membrane has recently attracted attention. In this process, since the liquid phase and the gas phase (vacuum) are separated by the membrane. The tower height is not subject to any restriction, unlike in the case of the vacuum degassing tower.
There is also a process comprising blowing N.sub.2 gas into water to increase the partial pressure of N.sub.2 gas in the water and thereby decrease the partial pressure of O.sub.2 gas in the water, to remove O.sub.2 gas from the water.
The above are physical processes for removing DO from water. Besides these, there are chemical processes utilizing a chemical reaction. For example, a reducing agent such as hydrazine (N.sub.2 H.sub.4), sodium sulfite (Na.sub.2 SO.sub.3) or the like is added to water to effect the following chemical reaction, whereby the DO in the water can be removed. EQU N.sub.2 H.sub.4 +O.sub.2 .fwdarw.N.sub.2 +2H.sub.2 O EQU 2Na.sub.2 SO.sub.3 +O.sub.2 .fwdarw.2Na.sub.2 SO.sub.4
In recent years, with the adoption of a higher density in semiconductor integrated circuits, the quality requirements for the ultrapure water used therefor have become increasingly higher. For example, as shown in Table 1, such water is required to contain electrolytes, fine particles, living microbes, etc. in extremely low concentrations, and DO in a concentration of 50 .mu.g/l or less.
TABLE 1 ______________________________________ Quality of Ultrapure Water and Integration of LSI Circuit Integration Item 1M 4M 16M ______________________________________ Resistivity (M.OMEGA.-cm at 25.degree. C.) &gt;17.5 &gt;18.0 &gt;18.0 Fine particles (counts/ml) 0.1 .mu.m &lt;20 0.08 .mu.m &lt;10 0.05 .mu.m &lt;10 Living microbes (counts/100 ml) &lt;10 &lt;5 &lt;1 TOC (.mu.g C/l) &lt;50 &lt;30 &lt;10 Silica (.mu.g SiO.sub.2 /l) &lt;10 &lt;3 &lt;1 DO (.mu.g O/l) &lt;100 &lt;50 &lt;50 ______________________________________
Achieving an extremely low concentration of DO with the conventional thermal degassing or vacuum degassing process alone is difficult not only from the principle of the process but also in view of the facility scale or treatment time required for the process. Therefore, in the case of, for example, feedwater for percolation type boilers, etc., a chemical process of injecting hydrazine (N.sub.2 H.sub.4) is used in combination with the above physical process, in order to reduce the DO in the feedwater to 100 .mu.g/l or less.
In removing DO by a chemical process, however, there remains the chemical reagent (e.g., N.sub.2 H.sub.4, Na.sub.2 SO.sub.3) added in excess relative to the amount of DO; the residual chemical reagent becomes a load to an ion exchange resin used in the ion exchange treatment, and moreover the SO.sub.4.sup.2- ion, etc. formed by the reaction of the chemical reagent with DO becomes a load to the ion exchange resin as well.
Further, the purging process by N.sub.2 requires the use of a large amount of N.sub.2 gas (1-2 Nm.sup.3 /m.sup.3), which is uneconomical.
As described above, in achieving an extremely low concentration of DO by conventional degassing processes, there have been problems; for example, a large facility is required, there occurs an increase in impurity ions, and a large amount of a chemical reagent or N.sub.2 gas is required.